This invention relates to noise suppression and in particular to that signature of noise that is occasioned in a coannular flow stream where there are spaced concentric pipes where the outer pipe extends beyond the inner one in the discharge path of both streams.
The invention can probably be best understood by considering the problem that it solved. While the problem was attendant a fan-jet engine, it will be appreciated by one skilled in the art that the problem can be inherent in other coannular flow streams and, hence, the solution will have equal applicability. By noting the graph depicted in FIG. 1 which shows 1/3 octave band sound pressure level (curve B) plotted against frequency, it is apparent that around the 4000 hertz frequency a spurious noise is present. This noise has been prognosticated as being occasioned by the large scale turbulence of the inner flow stream egressing from the inner pipe or splitter in the tail pipe of the fan-jet engine which forms a helical unsteady pressure field that impacted the outer pipe downstream thereof.
We have found that we can reduce or even eliminate this unwanted noise by destroying the coherence of the unsteady pressure field. By locating inwardly facing tabs in an unsymmetrical pattern around the inner pipe or splitter, we have found by actual tests that we can reduce this generated noise by as much as 5 dB. The effect on the spurious noise at 4000 hertz of reducing the helical unsteady pressure field with the invention described herein is illustrated in FIG. 1 (curve A).